1. Field of the Invention
This invention relates to a DC electric power supply device, and more particularly to a DC power supply device for reactive loads requiring strictly controlled direct current.
2. Description of the Prior Art
Accelerators of electrons, protons, etc. are used for research in nuclear physics. In the accelerators, electromagnets are used for controlling the electrons and protons, and the electromagnets are controlled by the current flowing in electromagnetic coils. The current in the coils must be changed rapidly, and precisely controlled as specified.
In a conventional magnetic coil for such an accelerator, the current is supplied by a DC power source and controlled by a transistor based on the measured current in the coil and the stipulated time-dependent current.
The voltage V imposed on the coil is given by EQU V=L ( di / dt ) +R i
where L and R denote reactance and resistance of the coil, respectively, i denotes the current in the coil and t denotes time.
Suppose the stipulated current i increases rapidly from zero to a positive value for a time period from t=0 to t=t1, and decreases rapidly to zero for a time period from t=t1 to t=t2, as shown in FIG. 2 (a). When the current i increases, di/dt becomes positive, and V becomes positive, which means V and i are in the same direction. However, when the current i decreases rapidly, di/dt becomes large in the negative direction, and V becomes negative, which means V and i are in opposite directions.
The current i can be controlled by a transistor. The voltage of the output of the DC power source minus the collector-emitter voltage V.sub.CE of the transistor is imposed on the coil. In this control, however, the power of V.sub.CE i is lost in the transistor. The voltage V.sub.CE becomes especially large and the power loss in the transistor becomes larger when the specified current decreases. Therefore, the dimensions of the transistor must be sufficiently large to dissipate the heat due to the large power loss in it, which results in expensive power supply devices.
Japanese Patent Disclosure (Kokai) No. 59-1520 discloses a DC power supply device for such magnetic coils. It has improve characteristics for increasing specified currents. However, when the current decreases, rapid and precise control cannot be achieved.